This invention relates generally to a carbonaceous pitch and, more specifically, to a novel, dormant mesophase pitch which is optically isotropic in nature but turns into an optically anisotropic material when subjected to shear forces. This invention is also directed to a process for the preparation of such a dormant mesophase pitch and a method of producing a carbon fiber from such a dormant mesophase pitch.
Pitches have been hitherto utilized as binders, impregnators and raw materials for graphite and the like carbonaceous materials. In addition, because of their high carbon content, pitches have become very important precursor materials for carbon fibers.
As precursor materials for carbon fibers, both optically isotropic and anisotropic pitches have been employed. Natural and synthetic pitches are generally isotropic in nature and afford isotropic carbon fibers with low-strength and low-modulus. On the other hand, anisotropic pitches can form carbon fibers having a strength and a modulus as high as those obtained from rayon or acrylic fibers. Therefore, the recent trend in the production of carbon fibers is towards the use of anisotropic pitches as starting materials.
Anisotropic pitches may be produced by thermal treatment of natural or synthetic pitches which are generally composed of condensed ring aromatics or average molecular weight of a few hundred or less and which are isotropic in nature. That is, when such isotropic pitches are heated to a temperature of about 350.degree.-450.degree. C., anisotropic, small spheres begin to appear in the matrix of isotropic pitch through cyclization, aromatization, polycondensation and the like reactions of the aromatics. These small spheres, which are considered to be liquid crystals of a nematic structure, are composed of relatively high molecular weight hydrocarbons having a polycyclic, condensed ring structure and a high aromaticity and are insoluble in quinoline. With the increase in heat treatment time or temperature, these small spheres gradually grow in size and coalesce with each other. As coalescence continues, the pitches turn into an anisotropic state as a whole, with the simultaneous increase in viscosity, and are finally converted into coke. The optically anisotropic, small spheres or their coalesced domains are called "mesophase" and pitches containing such material are termed as "mesophase piches". Conventional carbon fibers or anisotropic structures are produced by spinning a mesophase pitch, rendering the spun fibers infusible and carbonizing the infusible fibers, as disclosed in Japanese Examined Patent Publication No. 49-8634 and Japanese Published Unexamined Patent Applications Nos. 49-19127, 53-65425, 53-119326 and 54-160427.
The production of carbon fibers from mesophase pitches, however, has been found to involve certain difficulties in a mesophase pitch-forming step and in a spinning step. The fundamental problem is involved in the spinning step and is mainly ascribed to the fact that the mesophase components of the pitch have higher melting points and, in the molten state, higher viscosity than the components forming the isotropic matrix of the pitch. Namely, the mesophase pitch, when heated for spinning to a temperature so as to melt the isotropic matrix but not to melt the mesophase components, is thixotropic because of the presence of the solid-like phase mesophase components and, therefore, smooth spinning operation is seriously inhibited. If, on the other hand, the spinning temperature is raised to a temperature permitting the melting of the mesophase components, then the mesophase components, which are thermally unstable, gradually increase in their viscosity by polymerization and tend to form coke. Especially, in the case of the mesophase pitch having a high mesophase content, such coking proceeds very fast to the extent that the continuous spinning operation is considerably inhibited. Thus, although anisotropic carbon fibers derived from mesophase pitches have superior mechanical properties in comparison with isotropic carbon fibers obtained from isotropic pitches, the production of the anisotropic fibers inherently encounters with a problem in spinning step. In the conventional process for the production of anisotropic carbon fibers, therefore, it is essential to prepare mesophase pitches of a specific type having high spinnability.
As starting materials for other carbonaceous materials than carbon fibers, the known mesophase pitches are not satisfactory because they are not thermally stable and are viscous in the molten state. The known isotropic pitches are also not suitable enough as binders or impregnators since they fail to provide a high carbon yield.